A multifunctional electronic device has been used in electronic apparatus as one effective measure for reducing the size of the apparatus. As the multifunctional electronic device, for example, a ceramic multilayer module may be mentioned.
The ceramic multilayer module has a multilayer ceramic substrate. Wiring conductors are embedded in the multilayer ceramic substrate to serve an electrical connection function and/or to form at least one passive device such as a capacitor and/or an inductor, and in addition, various electronic devices are mounted on the multilayer ceramic substrate.
According to the structure described above, although being compact in size, the ceramic multilayer module can be configured to have a multifunctional performance, and by the use of this ceramic multilayer module, the electronic apparatus can be reduced in size.
In addition, besides reducing the size as described above, there has been an increasing request for the electronic apparatus to work satisfactorily in a higher frequency band. Under the situation described above, a multilayer ceramic substrate thereof is desired to have superior high frequency characteristics in a ceramic multilayer module used in a high frequency band. More particularly, an insulating ceramic sintered body used as an insulating ceramic layer which forms part of a multilayer structure of the multilayer ceramic substrate has been desired to have superior high frequency characteristics.
As an insulating ceramic composition to form an insulating ceramic sintered body that fulfills the desire described above, a composition disclosed, for example, in Japanese Unexamined Patent Application Publication No. 2000-344571 (Patent Document 1) may be mentioned. In Patent Document 1, a three-component insulating ceramic composition including forsterite, calcium titanate, and spinel has been disclosed. According to Patent Document 1, this insulating ceramic composition has a Qf value of 38,000 GHz or more, which is represented by frequency (GHz)/dielectric loss (tan δ), and a temperature coefficient of the dielectric constant of −80 to +40 ppm/° C. in a more preferable composition range.
When a multilayer ceramic substrate of the above-described ceramic multilayer module is manufactured, a firing step is carried out. In this firing step, wiring conductors provided for the multilayer ceramic substrate are also simultaneously fired.
In order to use the ceramic multilayer module in a high frequency band without causing any problems, the wiring conductors provided for the multilayer ceramic substrate must first have a low electrical resistance. Hence, a metal, such as copper or silver, having a low electrical resistance must be used as a conductive component contained in the wiring conductors.
However, the metal, such as copper or silver, has a relatively low melting point. Hence, in order to obtain the multilayer ceramic substrate by firing it simultaneously together with wiring conductors containing the metal described above, an insulating ceramic composition forming insulating ceramic layers of the multilayer ceramic substrate must be a composition which can be fired at a low temperature, such as 1,000° C. or less.
In the case of the insulating ceramic composition described in Patent Document 1, a firing temperature of 1,140 to 1,600° C. has been disclosed, and hence a condition in which firing can be performed at a temperature of 1,000° C. or less cannot be satisfied.
In addition, in order to obtain a multilayer ceramic substrate which can work satisfactorily in a higher frequency band and which can achieve a higher wiring-conductor density, the dielectric constant of insulating ceramic layers which form the multilayer ceramic substrate must be decreased. Incidentally, a concrete value of the relative dielectric constant of the insulating ceramic sintered body which is obtained by firing the insulating ceramic composition described in Patent Document 1 has not been disclosed.
An insulating ceramic composition has been disclosed in Patent Document 2 which can be fired at a temperature of 1,000° C. or less, and has a low relative dielectric constant, which has more superior high frequency characteristics, that is, in more particulars, which can control the temperature characteristics of resonant frequency to be small, and which can obtain a higher Q value.
In Patent Document 2, a glass ceramic composition has been proposed which includes a first ceramic powder containing forsterite as a primary component; a second ceramic powder including at least one selected from the group consisting of a ceramic powder containing calcium titanate as a primary component, a ceramic powder containing strontium titanate as a primary component, and a ceramic powder containing titanium oxide as a primary component; and a borosilicate glass. In the glass ceramic composition described above, the borosilicate glass contains 3 to 15 percent by weight of lithium in the form of Li2O, 30 to 50 percent by weight of magnesium in the form of MgO, 15 to 30 percent by weight of boron in the form of B2O3, 10 to 35 percent by weight of silicon in the form of SiO2, 6 to 20 percent by weight of zinc in the form of ZnO, and 0 to 15 percent by weight of aluminum in the form of Al2O3.
In recent years, the thickness of ceramic layers forming a base member of a multilayer ceramic electronic device has progressively decreased, and on the other hand, the number of signals having a high voltage to be handled has increased. Accordingly, a material forming the ceramic layers has been increasingly required to have higher electrical insulating reliability.
In addition, concomitant with the decrease in thickness of the multilayer ceramic electronic device, the multilayer ceramic electronic device itself is also required to have a high flexural strength.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2000-344571    Patent Document 2: International Publication WO 2005/082806 pamphlet